The present invention relates to a vacuum pump that generates negative pressure.
A vacuum pump has been known that includes a rotor and a housing, which accommodates and rotationally supports the rotor. Japanese Laid-Open Patent Publication No. 2008-157070 discloses an example of such a vacuum pump. The rotor of the vacuum pump is coupled to a camshaft of an internal combustion engine and thus rotates integrally with the camshaft. Rotation of the rotor changes the volume of the space in the housing and generates negative pressure.
The vacuum pump of the '070 publication includes an oil supply pipe located in the coupling section between the rotor and the camshaft. The oil supply pipe includes a first end, which is received by the rotor, and a second end, which is received by the camshaft. The rotor includes a first oil passage that communicates with the space in the housing. The camshaft includes an oil supply hole for supplying oil to the vacuum pump. The oil supply pipe connects the first oil passage to the oil supply hole of the camshaft.
The oil supply pipe can slide in the rotor and the camshaft. The end surface of the oil supply pipe that faces the rotor is in contact with a compressed return spring. The return spring constantly urges the oil supply pipe toward the camshaft. The end surface of the oil supply pipe that faces the camshaft receives pressure of the oil supplied through the oil supply hole. When the internal combustion engine is stopped and the oil pressure applied to the end surface of the camshaft is low, the urging force of the return spring holds the oil supply pipe in the first position near the camshaft. When the internal combustion engine is operated and the oil pressure applied to the end surface of the camshaft is high, the oil pressure moves the oil supply pipe against the urging force of the return spring and holds the oil supply pipe in the second position near the rotor.
The oil supply pipe includes an atmosphere communication hole that extends through the oil supply pipe in the radial direction to provide communication between the space in the oil supply pipe and the atmosphere. Movements of the oil supply pipe bring the space in the oil supply pipe into and out of communication with the atmosphere through the atmosphere communication hole. Specifically, when the internal combustion engine and the vacuum pump are stopped, the oil supply pipe is located in the first position. In this state, the space in the oil supply pipe communicates with the atmosphere through the atmosphere communication hole. That is, when the vacuum pump is stopped, the oil supply pipe provides communication between the space in the vacuum pump and the atmosphere.
When the vacuum pump is stopped, the negative pressure remaining in the space in the housing draws oil into the housing. However, when the space in the vacuum pump communicates with the atmosphere through the atmosphere communication hole as described above, air is drawn into the housing and releases the negative pressure. This reduces the amount of oil that is drawn into and remains in the vacuum pump.
When the internal combustion engine is operated and the vacuum pump is driven, the oil supply pipe is located in the second position. The section of the oil supply pipe that includes the atmosphere communication hole is located in the rotor. Thus, the atmosphere communication hole is closed, closing communication between the space in the oil supply pipe and the atmosphere.
Since the communication between the space in the housing and the atmosphere is closed when the vacuum pump is driven, air is not drawn into the housing through the atmosphere communication hole. This limits the amount of air discharged from the vacuum pump, thereby limiting air discharge noises.
When the vacuum pump of the '070 publication is stopped, the atmosphere communication hole also provides communication between the oil supply hole and the atmosphere, allowing air to flow into the oil supply hole through the atmosphere communication hole. Thus, when supply of oil is stopped, the oil in the oil supply passage tends to be discharged by its own weight. As a result, when the internal combustion engine starts again, the vacuum pump does not receive oil until the oil supply hole is filled with oil. This prevents prompt supply of oil to the vacuum pump through the oil supply hole.